<useLegacyJit> Element

Determines whether the common language runtime uses the legacy 64-bit JIT compiler for just-in-time compilation.

<configuration>
  <runtime>
    <useLegacyJit>

Syntax

<useLegacyJit enabled=0|1 />

The element name useLegacyJit is case-sensitive.

Attributes and elements

The following sections describe attributes, child elements, and parent elements.

Attributes

enabled attribute

Child elements

None

Parent elements

Remarks

Starting with .NET Framework 4.6, the common language runtime uses a new 64-bit compiler for Just-In-Time (JIT) compilation by default. In some cases, this may result in a difference in behavior from application code that was JIT-compiled by the previous version of the 64-bit JIT compiler. By setting the enabled attribute of the <useLegacyJit> element to 1, you can disable the new 64-bit JIT compiler and instead compile your app using the legacy 64-bit JIT compiler.

Instead of using a configuration file setting, you can enable the legacy 64-bit JIT compiler in two other ways:

• Setting an environment variable

  Set the COMPLUS_useLegacyJit environment variable to either 0 (use the new 64-bit JIT compiler) or 1 (use the older 64-bit JIT compiler):

  COMPLUS_useLegacyJit=0|1  
  

  The environment variable has global scope, which means that it affects all applications run on the machine. If set, it can be overridden by the application configuration file setting. The environment variable name is not case-sensitive.

• Adding a registry key

  You can enable the legacy 64-bit JIT compiler by adding a REG_DWORD value to either the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework or HKEY_CURRENT_USER\SOFTWARE\Microsoft\.NETFramework key in the registry. The value is named useLegacyJit. If the value is 0, the new compiler is used. If the value is 1, the legacy 64-bit JIT compiler is enabled. The registry value name is not case-sensitive.

  Adding the value to the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework key affects all apps running on the machine. Adding the value to the HKEY_CURRENT_USER\SOFTWARE\Microsoft\.NETFramework key affects all apps run by the current user. If a machine is configured with multiple user accounts, only apps run by the current user are affected, unless the value is added to the registry keys for other users as well. Adding the <useLegacyJit> element to a configuration file overrides the registry settings, if they're present.

Example

The following configuration file disables compilation with the new 64-bit JIT compiler and instead uses the legacy 64-bit JIT compiler.

<?xml version ="1.0"?>  
<configuration>  
  <runtime>  
    <useLegacyJit enabled="1" />  
  </runtime>  
</configuration>  

See also

• Configure apps by using configuration files
• <runtime> Element
• <configuration> Element
• Mitigation: New 64-bit JIT Compiler